The manufacture of a semiconductor involves a number of thermal cycles, in which a wafer, typically silicon, is heated from room temperature to a high temperature such as 900.degree. C., for example. Significantly higher or lower temperatures may be required depending upon the particular application. The wafer is heated relatively quickly, with a typical ramp rate of at least 100.degree. C. per second.
During such heating cycles, it is critically important that all points on the wafer remain at a uniform temperature relative to one another. If the temperature distribution across the wafer is non-uniform, thermal gradients will cause the crystal planes within the wafer to slip, thereby breaking the crystal lattice. A very small spatial movement, on the order of 0.2 .mu.m, may completely destroy the crystal lattice. Thermal gradients may also cause other damage, such as warpage or defect generation. Even in the absence of slippage, a non-uniform temperature distribution across the wafer may cause non-uniform performance-related characteristics, resulting in either inadequate performance of the particular wafer, or undesirable performance differences from wafer to wafer.
Thus, the industry defines a "process window", which is an acceptable temperature range in which the temperature of each portion of the wafer must be kept in order to maintain performance goals. In the past, a non-uniformity of no more than .+-.10.degree. C. across the wafer at all times during the thermal cycle was acceptable.
However, with the manufacture of increasingly high performance semiconductor computer chips, and as larger numbers of device features are required on increasingly compact chips, an increasingly uniform temperature distribution across the wafer is required at all times throughout the thermal cycles, i.e. both during ramp and at a process temperature, which is usually a constant temperature. Industry roadmaps indicate that for devices with 0.25 .mu.m spacing, at a process temperature of 1100.degree. C. a temperature uniformity of .+-.3.degree. C. (i.e. 3.degree. C.=3.sigma. where .sigma. is the standard deviation of the temperature distribution across the wafer) will be required, and temperature uniformity of .+-.1.degree. C. (3.sigma.) will be required for devices with 0.18 .mu.m spacing.
In addition, faster ramp rates, on the order of 400.degree. C. per second or higher, will be desired in the near future.
Conventional rapid thermal processing (RTP) techniques do not appear to be capable of achieving either the required degree of uniformity or the desired ramp rate.
One example of a conventional RTP technique includes rotating a wafer, and heating the wafer with a large number of tungsten-halogen lamps, each of which channels radiation toward the wafer surface through one of a large number of light pipes. Wafer temperature is measured with a comparatively small number of stationary pyrometers, each of which measures radiation thermally emitted by the wafer. Each measuring pyrometer is located at a different radial distance from the centre of the rotating wafer, so that the resulting temperature profile describes the average temperatures around a number of annular rings of the wafer, each annular ring corresponding to the radial distance of a particular measuring pyrometer. The resulting temperature versus time profile is then entered into a control computer, which employs a number of feedback control loops to control the power to the individual lamps or group of lamps associated with each pyrometer or sensor.
This technique has a disadvantage, in that it lacks the ability to detect or correct for temperature differences between any two points lying in the same annular ring, due to the constant rotation of the wafer relative to the pyrometers. Thus, while this technique is able to maintain a number of annular rings at relatively uniform average temperatures, it is not capable of either detecting or correcting for circumferential temperature differences. A mere 1% variation of absorption from one side of the wafer to the other may cause more than a 3.degree. C. temperature variation at 1050.degree. C. Thus, this technique is not suitable for the current industry requirements.
Also, to ensure accurate measurements, the plurality of pyrometers must be carefully calibrated, resulting in additional time and effort.
Modifying this technique for a non-rotating wafer would require a large increase in the number of pyrometers, which would lead to serious calibration difficulties, in addition to the added expense and difficulty of designing the hardware and software required to accommodate a large plurality of pyrometers and related control loops.
A further difficulty arises from reflection, by the walls of the process chamber, of radiation reflected or thermally emitted by the wafer. Such reflections may heat the wafer in a non-uniform manner, and may also produce measurement errors.
The substitution of a camera or CCD in this technique would not be practical, partly because the process hardware tends to obscure the view of the wafer, and partly because a camera or CCD would be particularly susceptible to errors induced by internally reflected radiation.
Furthermore, the use of a plurality of heat sources requires manual calibration of each such heat source, with the result that simple replacement of a burnt-out bulb may become a tedious and time-consuming process.
Moreover, the spectral distribution of tungsten-halogen heat sources may pose additional undesirable effects. Tungsten irradiance sources typically produce only 40% of their spectral energy below the 1.2 .mu.m band gap absorption of room-temperature silicon, resulting in an inefficient thermal cycle. Also, the wavelengths generated by tungsten sources may be sufficiently long to penetrate through a substrate side of the wafer and be non-uniformly absorbed by highly-doped features on a device side of the wafer, resulting in an increasingly non-uniform temperature distribution. Such an effect may be aggravated in devices involving insulating layers such as silicon on oxide (SOI). Irradiance fields produced by tungsten sources may be red-shifted as the power supplied to the source is decreased, resulting in even greater inefficiency and greater penetration of radiation into the device side.
In addition, as the temperature of silicon increases, it is able to absorb increasingly longer wavelengths of radiation. Thus, hotter areas of the wafer may absorb greater amounts of energy at the longer wavelengths produced by tungsten-halogen sources than cooler areas of the wafer, resulting in faster heating of the hotter areas and thermal runaway.
An additional problem arises from the slow thermal time constants of tungsten lamps. Fast ramp rates to desired process temperatures require fast feedback controls. For example, heating at 500.degree. C./sec to a process temperature with a uniformity of .+-.1.degree. C. ideally requires a response time of .+-.2 ms (.+-.1.degree. C./500.degree. C./sec), whereas tungsten lamps typically have much longer response times of fractions of a second.
Finally, this technique does not appear to be capable of achieving a ramp rate of 400.degree. C. per second which will soon be desired.
Thus, there is a need for a better heating device for semiconductor processing.